This adds a new temp-dir setting for controlling the temporary directory
without having to change the TMPDIR env var. This can be used to e.g.
use a path on a case-sensitive store on macOS for temporary files
without changing the TMPDIR var used by interactive shells or commands
invoked with `nix run`.
This also stops unsetting `TMPDIR` on darwin when the env var value
starts with `/var/folders/`, preferring instead to just do the check
when reading `TMPDIR`. This way the inherited `TMPDIR` env var is
preserved for child processes (such as interactive shells).
As a side effect this changes the behavior of `nix-build -o ''` to act
like `nix-build --no-out-link` instead of failing with an error caused
by trying to create a symlink at the cwd.
Fixes: #253
Fixes: #112
Change-Id: I9ee826323f2deca62854715a77ca7a373a948a29
Lily Ballard
3c8096e5cb